1. Field of the Invention
The present invention relates to a signal processing method of a walkie-talkie device, and more particularly, to a signal processing method with digital signal processing used for filtering signals.
2. Description of the Prior Art
A continuous tone-coded squelch system (CTCSS) has been widely applied to wireless transmission, and is used to make a plurality of users communicate with each other within a predetermined communication area. The CTCSS adopts a low-frequency CTCSS tone signal to distinguish signals transmitted via the same physical channel. For instance, a prior art walkie-talkie device utilizes the CTCSS to achieve group communication. Please refer to FIG. 1, which is a schematic diagram showing frequency bands used by the prior art CTCSS. As shown in FIG. 1, a band ranging from 62.5 Hz to 250 Hz is used to transmit the above-mentioned low-frequency CTCSS tone signal, and another band ranging from 300 Hz to 3.4 KHz is used to transmit speech signals spoken by a user. The operation of the CTCSS is described as follows. With regard to the prior art walkie-talkie device, 14 channels P1˜P14 generally are adopted to carry signals, and the 14 channels are physical channels. In addition, 38 CTCSS tone signals T1˜T38 individually corresponding to different frequencies are used. One of the 38 CTCSS tone signals T1˜T38 annexed to one physical channel generates a specific logical channel, and the 14 physical channels are capable of forming 532 (14*38) logical channels in total. When a speaker sets the walkie-talkie device with a physical channel P1 and a desired CTCSS tone signal T1, that is, the logical channel set by the speaker becomes P1 (T1) After the speaker presses a push-to-talk (PTT) button on the walkie-talkie device, the speaker is capable of outputting speech signals via the walkie-talkie device toward the predetermined communication area specified by the walkie-talkie device. If there are three listeners in the predetermined communication area, and the three listener set their own logical channels as P1 (T1) P1 (T38), P2 (T1) respectively. For the first listener with a logical channel P1 (T1) because his walkie-talkie device receives and transmits signals through the physical channel P1, the walkie-talkie device of the first listener will start receiving speech signals spoken by the speaker. In addition, the walkie-talkie device of the first listener judges that the CTCSS tone signal used by the speaker is T1. In other words, the speaker and the first listener both adopt the same logical channel P1 (T1). Therefore, the walkie-talkie device of the first listener then outputs the received speech signals via an audio speaker. The first listener is capable of hearing the speech signals spoken by the speaker. For the second listener with a logical channel P1 (T38) because his walkie-talkie device receives and transmits signals through the physical channel P1, the walkie-talkie device of the second listener will start receiving speech signals spoken by the speaker. However, the walkie-talkie device of the second listener judges that the CTCSS tone signal used by the speaker is not T1, but T38. In other words, the speaker and the second listener adopt different logical channels P1 (T1) and P1 (T38). Therefore, the walkie-talkie device of the second listener then does not output the received speech signals via an audio speaker. The second listener cannot hear the speech signals spoken by the speaker. For the third listener with a logical channel P2 (T1), because his walkie-talkie device receives and transmits signals through the physical channel P2, the walkie-talkie device of the third listener and that of the speaker use different physical channels. With the unmatched physical channel, the walkie-talkie device of the third listener cannot acknowledge an adequate signal strength indicated by a received signal strength indicator (RSSI). Therefore, the walkie-talkie device of the third listener will not receive any speech signals spoken by the speaker. That is, no speech signal is played by an audio speaker of the walkie-talkie device. The second and third listeners cannot hear any speech signals spoken by the speaker, that is, the walkie-talkie devices of the second and third listeners both detect the CTCSS tone signal related to the received speech signals for actuating a signal squelch function. To sum up, only the users using the same logical channel can communicate with each other to achieve group communication.
Please refer to FIG. 2, which is a block diagram of a prior art walkie-talkie device 10. The walkie-talkie device 10 has an antenna 11, a transceiver 12, a selector 14, a CTCSS decoder 16, a CTCSS encoder 18, a speech signal processor 20, a speaker 22, a microphone 24, and a controller 26. The walkie-talkie device 10 can receive and transmit radio frequency (RF) signals through the antenna 11. With regard to receiving RF signals, the transceiver 12 converts the high-frequency RF signal into a low-frequency baseband signal Rx, and transmits the baseband signal Rx to the selector 14. The selector 14 then outputs the baseband signal Rx from an output port A. The CTCSS decoder 16 determines frequency of a CTCSS tone signal according to the received baseband signal Rx. Generally speaking, the CTCSS decoder 16 has a low-pass filter (LPF) for extracting signals with frequencies ranging from 62.5 Hz to 250 Hz, and then the CTCSS decoder 16 judges the CTCSS tone signal related to the baseband signal Rx to decide whether the walkie-talkie device 10 and the baseband signal Rx use the same logical channel. At the same time, the decoder 16 sends a judgment result to the controller 26. If the walkie-talkie device 10 and the baseband signal Rx use the same logical channel, the controller 26 activates the speaker 22 to proceed following signal output operation. That is, the speech signal processor 20 has two analog filter circuits for extracting signals with frequencies ranging from 300 Hz to 3.4 KHz, and the extracted signals are played by the speaker 22. On the contrary, if the walkie-talkie device 10 and the baseband signal Rx use different logical channels, the controller 26 does not actuate the speech signal processor 20 and the speaker 22. The walkie-talkie device 10, therefore, does not output any speech signals transmitted by unmatched logical channels. With regard to transmitting RF signals, when the user presses the PTT button, the selector 14 will chose the input port B, and the controller 26 simultaneously actuates the microphone 24. Therefore, the speech signals spoken by the user are inputted into the speech signal processor 20. As mentioned above, the speech signal processor 20 uses filter circuits to extract signals with frequencies ranging from 300 Hz to 3.4 KHz, and the speech signal processor 20 outputs the extracted signals to the CTCSS encoder 18. Based on a CTCSS code (CTCSS tone signal with a specific frequency) set in the walkie-talkie device 10, the CTCSS encoder 18 adds a corresponding CTCSS tone signal in the extracted signals outputted from the speech signal processor 20 for forming a baseband signal Tx. In the end, the transceiver 12 converts the low-frequency baseband signal Tx into a high-frequency RF signal, and the RF signal is then outputted via the antenna 11.
The prior art walkie-talkie device 10 utilizes an analog signal processing means. While the antenna 11 receives the analog RF signal and the speaker 22 outputs the analog baseband signal Rx, the related signals are processed by the analog signal processing means to handle the CTCSS tone signal. Similarly, while the microphone 24 receives the analog speech signal and the antenna 11 outputs the analog baseband signal Tx, the related signals are processed by the analog signal processing means as well to handle the CTCSS tone signal. As mentioned above, CTCSS decoder 16 and the speech signal processor 20 both require analog filter circuits to extract desired frequency bands. However, the prior art filter circuit cannot have a sharp filtering quality owing to its hardware characteristic. For example, when 38 different CTCSS tone signals are uniformly distributed within the frequency band ranging from 62:5 Hz to 250 KHz, frequency difference between adjacent CTCSS tone signals is almost equal to 5 Hz. That is, the prior art CTCSS decoder 16 might mistake an adjacent CTCSS tone signal for the actual CTCSS tone signal owing to the above-mentioned bad filtering quality so that the reception performance for the desired speech signals is deteriorated. On the other hand, because the speech signal processor 20 cannot precisely extract speech signals with frequencies ranging from 300 Hz to 3.4 KHz, the CTCSS tone signal generated by the CTCSS encoder 18 is probably interfered with by the speech signals generated from the user. In addition, speech signals having frequencies greater than 3.4 KHz will be leak out and interfere with signals transmitted by other physical channels. Therefore, signal transmission for other users is greatly jammed by unwanted noises. When the walkie-talkie device 10 wants to provide the user with additional functions such as the signal scrambling, corresponding hardware circuits have to be added to the original walkie-talkie device 10. The hardware complexity and production cost are increased, and even the size and weight of the product are increased to weaken its competitiveness on the walkie-talkie market.